In the art it is desirable to produce elastomeric compounds exhibiting reduced hysteresis. Such elastomers, when compounded to form articles such as tires, power belts and the like, will show an increase in rebound, a decreased rolling resistance and less heat build-up when mechanical stresses are applied.
Previous attempts at preparing reduced hysteresis compounds have included high temperature mixing of the filler-rubber mixtures in the presence of selectively-reactive promoters to promote compounding material reinforcement; surface oxidation of the compounding materials; chemical modifications to the terminal end of polymers using tetramethyldiaminobenzophenone (Michler's ketone), tin coupling agents and the like and, surface grafting thereon. All of these approaches have focused upon increased interaction between the elastomer and the compounding materials.
It has also been recognized that carbon black, employed as a reinforcing filler in rubber compounds, should be well dispersed throughout the rubber in order to improve various physical properties. One example of the recognition is provided in published European Pat. Appln. EP 0 316 255 A2 which discloses a process for end capping polydienes by reacting a metal terminated polydiene with a capping agent such as a halogenated nitrile, a heterocyclic aromatic nitrogen-containing compound or an alkyl benzoate. Additionally, the application discloses that both ends of the polydiene chains can be capped with polar groups by utilizing functionalized initiators, such as lithium amides.
The present invention provides novel initiators for anionic polymerization, to form elastomers with functional groups derived from said initiators. The functional groups are incorporated into the polymer chain providing improved dispersability of carbon black throughout the elastomeric composition during compounding. As will be described hereinbelow, these initiators are compounds containing a moiety derived from a tertiary-amino allyllithium or a tertiary-amino xylyllithium.
Organolithium polymerization initiators are also known in the art. For example, U.S. Pat. No. 3,326,881 discloses phenyllithium initiator and U.S. Pat. No. 3,439,049 discloses an organolithium initiator prepared from a halophenol in a hydrocarbon medium. Phenyllithium initiators have proven to be unstable.
U.S. Pat. No. 4,015,061 is directed toward amino-functional initiators which polymerize diene monomers to form mono- or di-primary aryl amine-terminated diene polymers upon acid hydrolysis.
U.S. Pat. No. 4,914,147 discloses terminal modifying agents including dialkylamino-substituted aromatic vinyl compounds such as N,N'-dimethylamino benzophenone and p-dimethylamino styrene, in rubber compositions having reduced hysteresis characteristics. In U.S. Pat. No. 4,894,409, an amino group-containing monomer, such as 2-N,N-dimethylaminostyrene is polymerized to form an amino group-containing diene based polymer.
It is also known in the art to conduct polymerizations employing hydrocarbon lithium initiators at high temperatures. However, elevated temperatures make it more difficult to maintain the "living" ends or the polymer-lithium bonds needed for efficient polymerization and termination reactions. With known initiators it has been found that the lithium constituent will often combine with an available alpha-hydrogen atom, resulting in lithium hydride, especially at elevated temperatures, thereby destroying the initiator and causing additional harmful side reactions. Hence, high temperature polymerizations have proven to be difficult to maintain and difficult to terminate efficiently.
A need exists therefore, for a polymerization initiator which when employed in an anionic polymerization, will result in a polymer chain having a functional group derived from the initiator. A need also exists for such an initiator which will perform effectively at high temperatures resulting in narrow molecular weight distribution polymers and retention of "living" ends.